The present invention relates to a power steering apparatus for reducing an operating force (steering force) of a steering wheel and, more particularly, to an improvement in an electric power steering apparatus comprising an electric motor as an auxiliary steering force generating means.
A hydraulic power steering apparatus has been mainly employed as a conventional power steering apparatus. However, various types of electric power steering system using an electric motor or the like have recently been proposed in, e.g., Japanese Patent Laid-Open No. 61-226362. More specifically, a hydraulic power steering apparatus requires a large number of components, such as a hydraulic cylinder for generating an auxiliary steering force, a hydraulic pump, a flow path selection valve, and a hydraulic pipe system for connecting these components. In addition, the arrangement of each component is complex, and hence high process precision and the like are required. Therefore, the manufacture and assembly of the apparatus are cumbersome, and moreover, the size and weight of the overall apparatus are increased, thus posing problems, such as an increase in cost. For this reason, a great deal of attention has been paid to an electric power steering apparatus comprising an electric motor, as an auxiliary steering force generating means, which can be used by connecting it to a built-in battery, a controller, and the like by using simple electric wiring. Since the arrangement of an apparatus of this type can be simplified as compared with the above-described hydraulic power steering apparatus, a small, compact power steering apparatus can be realized.
Employment of an electric motor as an auxiliary steering power generating means in a power steering apparatus, however, poses various problems in terms of the assembly structure of the motor with respect to a steering system, the arrangement and performance of each component for operation control of the motor, and the like. For this reason, practical use of such a type of apparatus has not been realized.
One of the problems in the electric power steering apparatus is associated with the coupling structure of an input shaft on the steering wheel side, which constitutes a steering shaft as a main shaft, with an output shaft on the wheel side. Generally, the input and output shafts constituting the steering shaft are coupled through a torsion bar to each other so as to be pivotally displaced from each other because a steering angle, a steering force, and the like upon steering operation must be detected. In addition, a detecting mechanism capable of detecting a relative pivotal displacement due to torsion of the torsion bar between the shafts is arranged at the coupling portion, so that drive control of the motor for applying an auxiliary steering force to a portion of the output shaft system can be performed by using a signal from the detecting mechanism. The simplest arrangement is that the input and output shafts are arranged so as to cause their end portions to oppose each other, and a relative displacement between rotary detecting members mounted on the shafts is detected. In such an arrangement, the size of the apparatus in the axial direction is increased, and hence the size of the overall apparatus is inevitably increased.
On the other hand, an electric power steering apparatus of this type is required to have a simple overall arrangement allowing easy manufacture, assembly, and the like, and a decrease in size and weight of the overall apparatus. In addition, a demand has arisen for a compact, low-cost apparatus which is excellent in performance and mountability. Such a demand has become strong because an apparatus of this type is employed even in small cars and the like in which the problem of space is serious.
In addition, such an electric power steering apparatus is required to be arranged such that a motor is properly driven/controlled by simply and properly detecting various vehicle travel conditions, such as a steering force, a steering angle, and a vehicle speed corresponding to steering wheel operation of a driver, thereby obtaining an auxiliary steering force as needed. More specifically, during a stationary turn in a vehicle stop state, or during steering at a low vehicle speed, a very small steering force is required. However, during high-speed travel, proper rigidity, i.e., a sense of heaviness is required. In order to satisfy such a requirement, such an apparatus must be driven/controlled in accordance with a vehicle speed, a steering force, and a steering angle. Therefore, a detecting mechanism for such an operation must be arranged with high precision.
Various conventional arrangements are known as such a detecting mechanism. However, each of them has problems in terms of structure and performance. This is conspicuous in, e.g., a steering force detecting mechanism for detecting a steering request from a driver with respect to a steering wheel. Thus, various problems are posed especially in terms of a detecting method, a portion to be detected, and an arrangement position and detection performance of the detecting mechanism. As the above-described steering force detecting mechanism, a mechanism constituted by a noncontact type torque sensor, a signal processing circuit, and the like may be incorporated in a steering gear body while being interposed between input and output shafts constituting a steering shaft. This detecting mechanism requires a large number of components, and moreover, it must be mounted on the output shaft or the body side in a pivotal or stationary state. Therefore, assembly and the like become cumbersome and complicated. In addition, since mounting precision among components greatly influences the detecting precision of the sensor described above, relative positioning of these components is cumbersome, thus posing problems in terms of workability and operation reliability. Therefore, a strong demand has arisen for a countermeasure against these problems.